Rubber hydrochloride in sheet form is an article of commerce having a wide variety of known uses. This material is produced by the treatment of a continuous elongated sheet or strip of latex rubber with a solution of hydrogen chloride. This normally results in initial softening, swelling and tackiness of the rubber sheet with consequent elongation and difficulty of handling the sheet without damage, either due to its low tensile strength at that stage or due to adhesion of the sheet to itself while in the tacky state. It has previously been proposed to avoid such self-adhesion of the sheet by leading it back and forth in a zigzag course over rollers, for example, in an enclosed reaction zone with the initial movement of the sheet being downwards to minimize tension in advancing the sheet. It has been found, however, that this means of handling the advancing sheet is suitable for only about the first 5 minutes of the initial reaction period while the sheet is swollen, tacky and limp. As the initial hydrochlorination reaction proceeds during the next 10 minutes, the sheet begins to stiffen and it becomes more and more difficult to pass the sheet around the zigzag path as it becomes more and more inflexible. Moreover, the stiffness of the sheet continues to increase during the remainder of the hydrochlorination reaction up to the point of complete hydrochlorination which is reached after about 30 to 60 minutes of treatment. This increased stiffness also makes it difficult to withdraw the rubber hydrochloride from the apparatus in the form of an elongated tape or strip of finished rubber hydrochloride.
It has also been customary in the prior art processes to remove excess entrained, unreacted hydrogen chloride solution by vaporization in a heated zone. However, such evaporative removal has the disadvantages that the high heat required for the evaporation and residual hydrogen chloride degrade the rubber hydrochloride causing it to discolor. Furthermore, no practical means has yet been devised to treat the evaporated reaction solution, e.g. of hydrogen chloride in ethyl acetate.
It is a primary object of the present invention, therefore, to provide an improved process for the production of rubber hydrochloride sheet which avoids the handling problems attendant on prior art processes, and which produces a superior product free from contaminates and discoloration.
It is a more particular object of the invention to provide a process which insures thorough immersion and treatment of the rubber sheet while avoiding self-adhesion of the sheet while it is in tacky condition.
It is another object of the invention to provide improved handling of the rubber sheet during hydrochlorination thereof to avoid slack in the sheet due to the initial swelling thereof.
It is still another object of the invention to avoid subjecting the rubber sheet to a zigzag passage after it becomes too stiff for such handling.
It is yet another object of the invention to avoid discoloration of the rubber hydrochloride sheet by residual reaction solution and/or overheating during the removal of reaction solution or other contaminants.
It is another object of the invention to avoid the introduction of water or contaminants from the neutralization of hydrogen chloride.
It is a further object of the invention to provide rubber hydrochloride sheet of uniform thickness by calendering the sheet at an elevated temperature, preferably before it cools from the drying operation.